Modified start sequence of a gas turbine engine

ABSTRACT

A system for starting a gas turbine engine of an aircraft is provided. The system includes a pneumatic starter motor, a discrete starter valve switchable between an on-state and an off-state, and a controller operable to perform a starting sequence for the gas turbine engine. The starting sequence includes alternating on and off commands to an electromechanical device coupled to the discrete starter valve to achieve a partially open position of the discrete starter valve to control a flow from a starter air supply to the pneumatic starter motor to drive rotation of a starting spool of the gas turbine engine below an engine idle speed.

BACKGROUND

This disclosure relates to gas turbine engines, and more particularly toan apparatus, system and method for modifying a start sequence of thegas turbine engine.

Gas turbine engines are used in numerous applications, one of which isfor providing thrust to an airplane. When the gas turbine engine of anairplane has been shut off for example, after an airplane has landed atan airport, the engine is hot and due to heat rise, the upper portionsof the engine will be hotter than lower portions of the engine. Whenthis occurs thermal expansion may cause deflection of components of theengine which may result in a “bowed rotor” condition. If a gas turbineengine is in such a “bowed rotor” condition it is undesirable to restartor start the engine.

Accordingly, it is desirable to provide a method and/or apparatus formitigating a “bowed rotor” condition.

BRIEF DESCRIPTION

In an embodiment, a system for starting a gas turbine engine of anaircraft is provided. The system includes a pneumatic starter motor, adiscrete starter valve switchable between an on-state and an off-state,and a controller operable to perform a starting sequence for the gasturbine engine. The starting sequence includes rapidly alternating onand off commands to an electromechanical device coupled to the slowermoving, discrete starter valve to achieve a partially open position ofthe discrete starter valve to control a flow from a starter air supplyto the pneumatic starter motor to drive rotation of a starting spool ofthe gas turbine engine to a dry motoring speed below a shaft resonancespeed which is also below an engine idle speed.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the electromechanical device has a cycle time definedbetween an off-command to an on-command to the off-command that is atmost half of a movement time for the discrete starter valve totransition from fully closed to fully open.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the electromechanical device is a solenoid that positionsthe discrete starter valve based on intermittently supplied electricpower.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the electromechanical device is an electric valvecontrolling muscle air to adjust the position of the discrete startervalve.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the controller modulates the on and off commands to theelectromechanical device to further open the discrete starter valve andincrease a rotational speed of the starting spool.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude an electronic engine control system that includes a memory forrecording a current heat state of the gas turbine engine at shutdown andfor recording a shutdown time of the gas turbine engine. The electronicengine control system further includes a risk model for determining atime period (t_(motoring)) for motoring the gas turbine engine at abouta predetermined speed range N_(target+/−)N where the predetermined speedis less than a speed used to start the gas turbine engine and wheret_(motoring) is a function of the heat state recorded at engine shutdownand an elapsed time of an engine start request relative to the previousshutdown time.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the time period (t_(motoring)) is calculated automaticallyduring a start of the gas turbine engine.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the controller modulates a duty cycle of the discretestarter valve via pulse width modulation.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the predetermined speed N_(target) is within apredetermined speed range N_(targetMin) to N_(targetMax) that is usedregardless of the calculated time period t_(motoring).

According to an embodiment, a gas turbine engine and a system forcontrolling a start sequence of the gas turbine engine includes anelectronic engine control system, a thermal model, memory, a model fordetermining a time period (t_(motoring)), and a controller. The thermalmodel is resident upon the electronic engine control system andconfigured to synthesize a heat state of the gas turbine engine. Thememory is for recording the current heat state of the gas turbine engineat shutdown and for recording a shutdown time of the gas turbine engine.The model for determining the time period is for motoring the gasturbine engine at a predetermined speed N_(target) wherein thepredetermined speed is less than a speed used to start the gas turbineengine and wherein t_(motoring) is a function of the heat state recordedat engine shutdown and an elapsed time of an engine start requestrelative to a previous shutdown time. The controller is for modulating astarter valve of a starter of the gas turbine engine in order tomaintain the gas turbine engine within a predetermined speed range ofN_(targetMin) to N_(targetMax) for homogenizing engine temperatures.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the time period (t_(motoring)) is calculated automaticallyduring a start of the gas turbine engine.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the controller modulates a duty cycle of the starter valvevia pulse width modulation.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the starter valve controls the flow of an air supply intothe starter of the gas turbine engine.

According to an embodiment, a method for providing a start sequence of agas turbine engine includes determining a heat state of the gas turbineengine via an engine thermal model. The method includes storing the heatstate of the gas turbine engine at shutdown. The method further includesrecording a time of the engine shutdown and using a risk model todetermine a motoring time period t_(motoring) for a start sequence ofthe gas turbine engine, wherein the risk model uses the recorded time ofthe engine shutdown and the stored heat state of the gas turbine engineat shut down to determine the motoring time period t_(motoring).

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the gas turbine engine is motored at a predetermined speedrange of N_(targetMin) to N_(targetMax) during the motoring time period,which is less than a normal idle start speed N2.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude dynamically varying a position of a starter valve during themotoring time period in order to motor the gas turbine engine at thepredetermined speed range of N_(targetMin) to N_(targetMax).

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the risk model is located on an electronic control of asystem programmed to automatically implement the motoring time period.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the heat state is determined by a first thermal model anda second thermal model each being resident upon the electronic enginecontrol system.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude providing a time of an engine start request and an inlet airtemperature to the risk model when the risk model determines themotoring time period.

In addition to one or more of the features described above, or as analternative to any of the foregoing embodiments, further embodiments mayinclude where the motoring time period determines the duration of amodified start sequence of the gas turbine engine and the modified startsequence requires motoring of the gas turbine engine at a predeterminedspeed N_(target) during the motoring time period without introduction offuel and an ignition source to the gas turbine engine, where thepredetermined speed N_(target) is less than a normal idle start speedN2.

A technical effect of the apparatus, systems and methods is achieved byusing a start sequence for a gas turbine engine as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the present disclosure isparticularly pointed out and distinctly claimed in the claims at theconclusion of the specification. The foregoing and other features, andadvantages of the present disclosure are apparent from the followingdetailed description taken in conjunction with the accompanying drawingsin which:

FIG. 1 is a cross-sectional view of a gas turbine engine;

FIG. 2 is a schematic illustration of a starting system for a gasturbine engine in accordance with an embodiment of the disclosure;

FIG. 3 is a schematic illustration of a starting system for a gasturbine engine in accordance with another embodiment of the disclosure;

FIG. 4 is a block diagram of a system for bowed rotor start mitigationin accordance with an embodiment of the disclosure;

FIG. 5 is a flow chart illustrating a method of bowed rotor startmitigation of a gas turbine engine in accordance with an embodiment ofthe disclosure;

FIG. 6 is a graph illustrating a bowed rotor risk score with respect totime in accordance with an embodiment of the disclosure;

FIG. 7 is a graph illustrating a normal or cooled engine start versus amodified engine start in accordance with an embodiment of thedisclosure;

FIG. 8 is a graph illustrating examples of various vibration levelprofiles of an engine in accordance with an embodiment of thedisclosure;

FIG. 9 is a schematic illustration of a high spool gas path with astraddle-mounted spool in accordance with an embodiment of thedisclosure;

FIG. 10 is a schematic illustration of a high spool gas path with anoverhung spool in accordance with an embodiment of the disclosure;

FIG. 11 is a graph illustrating commanded starter valve opening withrespect to time in accordance with an embodiment of the disclosure; and

FIG. 12 is a graph illustrating a target rotor speed profile of a drymotoring profile and an actual rotor speed versus time in accordancewith an embodiment of the disclosure.

DETAILED DESCRIPTION

Various embodiments of the present disclosure are related to a bowedrotor start mitigation system in a gas turbine engine. Embodiments caninclude using a starter valve to control a rotor speed of a startingspool of the gas turbine engine to mitigate a bowed rotor conditionusing a dry motoring process. During dry motoring, the starter valve canbe actively adjusted to deliver air pressure from an air supply to anengine starting system that controls starting rotor speed. Dry motoringmay be performed by running an engine starting system at a lower speedwith a longer duration than typically used for engine starting whiledynamically adjusting the starter valve to maintain the rotor speedand/or follow a dry motoring profile. Some embodiments increase therotor speed of the starting spool to approach a critical rotor speedgradually and as thermal distortion is decreased they then acceleratebeyond the critical rotor speed to complete the engine starting process.The critical rotor speed refers to a major resonance speed where, if thetemperatures are unhomogenized, the combination of a bowed rotor andsimilarly bowed casing and the resonance would lead to high amplitudeoscillation in the rotor and high rubbing of blade tips on one side ofthe rotor, especially in the high pressure compressor if the rotor isstraddle-mounted.

A dry motoring profile for dry motoring can be selected based on variousparameters, such as a modeled temperature value of the gas turbineengine used to estimate heat stored in the engine core when a startsequence is initiated and identify a risk of a bowed rotor. The modeledtemperature value alone or in combination with other values (e.g.,measured temperatures) can be used to calculate a bowed rotor riskparameter. For example, the modeled temperature can be adjusted relativeto an ambient temperature when calculating the bowed rotor riskparameter. The bowed rotor risk parameter may be used to take a controlaction to mitigate the risk of starting the gas turbine engine with abowed rotor. The control action can include dry motoring consistent withthe dry motoring profile. In some embodiments, a targeted rotor speedprofile of the dry motoring profile can be adjusted as dry motoring isperformed. As one example, if excessive vibration is detected as therotor speed rises and approaches but remains well below the criticalrotor speed, then the rate of rotor speed increases scheduled in the drymotoring profile can be reduced (i.e., a shallower slope) to extend thedry motoring time. Similarly, if vibration levels are observed below anexpected minimum vibration level as the rotor speed increases, the drymotoring profile can be adjusted to a higher rate of rotor speedincreases to reduce the dry motoring time.

A full authority digital engine control (FADEC) system or other systemmay send a message to the cockpit to inform the crew of an extended timestart time due to bowed rotor mitigation actions prior to completing anengine start sequence. If the engine is in a ground test or in a teststand, a message can be sent to the test stand or cockpit based on thecontrol-calculated risk of a bowed rotor. A test stand crew can bealerted regarding a requirement to keep the starting spool of the engineto a speed below the known resonance speed of the rotor in order tohomogenize the temperature of the rotor and the casings about the rotorwhich also are distorted by temperature non-uniformity.

Monitoring of vibration signatures during the engine starting sequencecan also or separately be used to assess the risk that a bowed rotorstart has occurred due to some system malfunction and then directmaintenance, for instance, in the case of suspected outer air seal rubespecially in the high compressor. Vibration data for the engine canalso be monitored after bowed rotor mitigation is performed during anengine start sequence to confirm success of bowed rotor mitigation. Ifbowed rotor mitigation is unsuccessful or determined to be incomplete bythe FADEC, resulting metrics (e.g., time, date, global positioningsatellite (GPS) coordinates, vibration level vs. time, etc.) of theattempted bowed rotor mitigation can be recorded and/or transmitted todirect maintenance

Referring now to FIG. 1, a schematic illustration of a gas turbineengine 10 is provided. The gas turbine engine 10 has among othercomponents a fan through which ambient air is propelled into the enginehousing, a compressor for pressurizing the air received from the fan anda combustor wherein the compressed air is mixed with fuel and ignitedfor generating combustion gases. The gas turbine engine 10 furthercomprises a turbine section for extracting energy from the combustiongases. Fuel is injected into the combustor of the gas turbine engine 10for mixing with the compressed air from the compressor and ignition ofthe resultant mixture. The fan, compressor, combustor, and turbine aretypically all concentric about a central longitudinal axis of the gasturbine engine 10. Thus, thermal deflection of the components of the gasturbine engine 10 may create the aforementioned bowing or “bowed rotor”condition along the common central longitudinal axis of the gas turbineengine 10 and thus it is desirable to clear or remove the bowedcondition prior to the starting or restarting of the gas turbine engine10.

FIG. 1 schematically illustrates a gas turbine engine 10 that can beused to power an aircraft, for example. The gas turbine engine 10 isdisclosed herein as a multi-spool turbofan that generally incorporates afan section 22, a compressor section 24, a combustor section 26 and aturbine section 28. The fan section 22 drives air along a bypassflowpath while the compressor section 24 drives air along a coreflowpath for compression and communication into the combustor section 26then expansion through the turbine section 28. Although depicted as aturbofan gas turbine engine in the disclosed non-limiting embodimentwith two turbines and is sometimes referred to as a two spool engine, itshould be understood that the concepts described herein are not limitedto use with turbofans as the teachings may be applied to other types ofturbine engines including three-spool architectures. In both of thesearchitectures the starting spool is that spool that is located aroundthe combustor, meaning the compressor part of the starting spool isflowing directly into the combustor and the combustor flows directlyinto the turbine section.

The engine 10 generally includes a low speed spool 30 and a high speedspool 32 mounted for rotation about an engine central longitudinal axisA relative to an engine static structure 36 via several bearing systems38. It should be understood that various bearing systems 38 at variouslocations may alternatively or additionally be provided.

The low speed spool 30 generally includes an inner shaft 40 thatinterconnects a fan 42, a low pressure compressor 44 and a low pressureturbine 46. The inner shaft 40 is connected to the fan 42 through ageared architecture 48 to drive the fan 42 at a lower speed than the lowspeed spool 30 in the example of FIG. 1. The high speed spool 32includes an outer shaft 50 that interconnects a high pressure compressor52 and high pressure turbine 54. The high speed spool 32 is alsoreferred to as a starting spool, as an engine starting system drivesrotation of the high speed spool 32. A combustor 56 is arranged betweenthe high pressure compressor 52 and the high pressure turbine 54. Amid-turbine frame 57 of the engine static structure 36 is arrangedgenerally between the high pressure turbine 54 and the low pressureturbine 46. The mid-turbine frame 57 further supports bearing systems 38in the turbine section 28. The inner shaft 40 and the outer shaft 50 areconcentric and rotate via bearing systems 38 about the engine centrallongitudinal axis A which is collinear with their longitudinal axes.

The core airflow is compressed by the low pressure compressor 44 thenthe high pressure compressor 52, mixed and burned with fuel in thecombustor 56, then expanded over the high pressure turbine 54 and lowpressure turbine 46. The mid-turbine frame 57 includes airfoils 59 whichare in the core airflow path. The turbines 46, 54 rotationally drive therespective low speed spool 30 and high speed spool 32 in response to theexpansion.

A number of stations for temperature and pressuremeasurement/computation are defined with respect to the gas turbineengine 10 according to conventional nomenclature. Station 2 is at aninlet of low pressure compressor 44 having a temperature T2 and apressure P2. Station 2.5 is at an exit of the low pressure compressor 44having a temperature T2.5 and a pressure P2.5. Station 3 is at an inletof the combustor 56 having a temperature T3 and a pressure P3. Station 4is at an exit of the combustor 56 having a temperature T4 and a pressureP4. Station 4.5 is at an exit of the high pressure turbine 54 having atemperature T4.5 and a pressure P4.5. Station 5 is at an exit of the lowpressure turbine 46 having a temperature T5 and a pressure P5.Temperatures in embodiments may be measured and/or modeled at one ormore stations 2-5. Measured and/or modeled temperatures can benormalized to account for hot day/cold day differences. For instance,measured temperature T2 can be used as an ambient temperature and amodeled temperature (e.g., T3) can be normalized by subtracting measuredtemperature T2.

Although FIG. 1 depicts one example configuration, it will be understoodthat embodiments as described herein can cover a wide range ofconfigurations. For example, embodiments may be implemented in aconfiguration that is described as a “straddle-mounted” spool 32A ofFIG. 9. This configuration places two bearing compartments 37A and 39A(which may include a ball bearing and a roller bearing respectively),outside of the plane of most of the compressor disks of high pressurecompressor 52A and at outside at least one of the turbine disks of highpressure turbine 54A. In contrast with a straddle-mounted spoolarrangement, other embodiments may be implemented using an over-hungmounted spool 32B as depicted in FIG. 10. In over-hung mounted spool32B, a bearing compartment 37B is located forward of the first turbinedisk of high pressure turbine 54B such that the high pressure turbine54B is overhung, and it is physically located aft of its main supportingstructure. The use of straddle-mounted spools has advantages anddisadvantages in the design of a gas turbine, but one characteristic ofthe straddle-mounted design is that the span between the bearingcompartments 37A and 39A is long, making the amplitude of the high spotof a bowed rotor greater and the resonance speed that cannot betransited prior to temperature homogenization is lower. For any thrustrating, the straddle mounted arrangement, such as straddle-mounted spool32A, gives Lsupport/Dhpt values that are higher, and the overhungmounted arrangement, such as overhung spool 32B, can be as much as 60%of the straddle-mounted Lsupport/Dhpt. Lsupport is the distance betweenbearings (e.g., between bearing compartments 37A and 39A or betweenbearing compartments 37B and 39B), and Dhpt is the diameter of the lastblade of the high pressure turbine (e.g., high pressure turbine 54A orhigh pressure turbine 54B). As one example, a straddle-mounted enginestarting spool, such as straddle-mounted spool 32A, with a rollerbearing at bearing compartment 39A located aft of the high pressureturbine 54A may be more vulnerable to bowed rotor problems since theLsupport/Dhpt ranges from 1.9 to 5.6. FIGS. 9 and 10 also illustrate astarter 120 interfacing via a tower shaft 55 with the straddle-mountedspool 32A proximate high compressor 52A and interfacing via tower shaft55 with the overhung mounted spool 32B proximate high compressor 52B aspart of a starting system.

Turning now to FIG. 2, a schematic of a starting system 100 for the gasturbine engine 10 of FIG. 1 is depicted according to an embodiment. Thestarting system 100 is also referred to generally as a gas turbineengine system. In the example of FIG. 2, the starting system 100includes a controller 102 which may be an electronic engine control ofan electronic engine control system, such as a dual-channel FADEC,and/or engine health monitoring unit. In an embodiment, the controller102 may include memory to store instructions that are executed by one ormore processors. The executable instructions may be stored or organizedin any manner and at any level of abstraction, such as in connectionwith a controlling and/or monitoring operation of the engine 10 ofFIG. 1. The one or more processors can be any type of central processingunit (CPU), including a general purpose processor, a digital signalprocessor (DSP), a microcontroller, an application specific integratedcircuit (ASIC), a field programmable gate array (FPGA), or the like.Also, in embodiments, the memory may include random access memory (RAM),read only memory (ROM), or other electronic, optical, magnetic, or anyother computer readable medium onto which is stored data and controlalgorithms in a non-transitory form.

The starting system 100 can also include a data storage unit (DSU) 104that retains data between shutdowns of the gas turbine engine 10 ofFIG. 1. The DSU 104 includes non-volatile memory and retains databetween cycling of power to the controller 102 and DSU 104. Acommunication link 106 can include an aircraft and/or test standcommunication bus to interface with aircraft controls, e.g., a cockpit,various onboard computer systems, and/or a test stand.

A motoring system 108 is operable to drive rotation of a starting spool(e.g., high speed spool 32) of the gas turbine engine 10 of FIG. 1.Either or both channels of controller 102 can alternate on and offcommands to an electromechanical device 110 coupled to a discretestarter valve 116A to achieve a partially open position of the discretestarter valve 116A to control a flow from a starter air supply 114 (alsoreferred to as air supply 114) through a transfer duct 118 to an airturbine starter 120 (also referred to as starter 120 or pneumaticstarter motor 120) to drive rotation of a starting spool of the gasturbine engine 10 below an engine idle speed. The air supply 114 (alsoreferred to as starter air supply 114) can be provided by any knownsource of compressed air, such as an auxiliary power unit or groundcart.

The controller 102 can monitor a speed sensor, such as speed pickup 122that may sense the speed of the engine rotor through its connection to agearbox 124 which is in turn connected to the high speed spool 32 viatower shaft 55 (e.g., rotational speed of high speed spool 32) or anyother such sensor for detecting or determining the speed of the gasturbine engine 10 of FIG. 1. The starter 120 may be coupled to thegearbox 124 of the gas turbine engine 10 of FIG. 1 directly or through atransmission such as a clutch system (not depicted). The controller 102can establish a control loop with respect to rotor speed to adjustpositioning of the discrete starter valve 116A.

The discrete starter valve 116A is an embodiment of a starter valve thatis designed as an on/off valve which is typically commanded to eitherfully opened or fully closed. However, there is a time lag to achievethe fully open position and the fully closed position. By selectivelyalternating an on-command time with an off-command time through theelectromechanical device 110, intermediate positioning states (i.e.,partially opened/closed) can be achieved. The controller 102 canmodulate the on and off commands (e.g., as a duty cycle using pulsewidth modulation) to the electromechanical device 110 to further openthe discrete starter valve 116A and increase a rotational speed of thestarting spool of the gas turbine engine 10 of FIG. 1. In an embodiment,the electromechanical device 110 has a cycle time defined between anoff-command to an on-command to the off-command that is at most half ofa movement time for the discrete starter valve 116A to transition fromfully closed to fully open. Pneumatic lines 112A and 112B or amechanical linkage (not depicted) can be used to drive the discretestarter valve 116A between the open position and the closed position.The electromechanical device 110 can be a solenoid that positions thediscrete starter valve 116A based on intermittently supplied electricpower as commanded by the controller 102. In an alternate embodiment,the electromechanical device 110 is an electric valve controlling muscleair to adjust the position of the discrete starter valve 116A ascommanded by the controller 102.

In the example of FIG. 2, the engine also includes a vibrationmonitoring system 126. The vibration monitoring system 126 includes atleast one vibration pickup 128, e.g., an accelerometer, operable tomonitor vibration of the gas turbine engine 10 of FIG. 1. Vibrationsignal processing 130 can be performed locally with respect to thevibration pickup 128, within the controller 102, or through a separatevibration processing system, which may be part of an engine healthmonitoring system to acquire vibration data 132. Alternatively, thevibration monitoring system 126 can be omitted in some embodiments.

Similar to FIG. 2, FIG. 3 is a schematic illustration of a startingsystem 100A for the gas turbine engine 10 of FIG. 1 in accordance withanother embodiment. The starting system 100A includes controller 102that controls motoring system 108A, as an alternate embodiment of themotoring system 108 of FIG. 2. Rather than using an electromechanicaldevice 110 coupled to a discrete starter valve 116A to achieve apartially open position of the discrete starter valve 116A of FIG. 2,the motoring system 108A of FIG. 3 uses a variable position startervalve 116B. In FIG. 3, either or both channels of controller 102 canoutput a valve control signal 150 operable to dynamically adjust a valveangle of the variable position starter valve 116A that selectivelyallows a portion of the air supply 114 to pass through the variableposition starter valve 116B and transfer duct 118 to air turbine starter120. The variable position starter valve 116B is a continuous/infinitelyadjustable valve that can hold a commanded valve angle, which may beexpressed in terms of a percentage open/closed and/or an angular value(e.g., degrees or radians). Performance parameters of the variableposition starter valve 116B can be selected to meet dynamic responserequirements of the starting system 100A. For example, in someembodiments, the variable position starter valve 116B has a responserate of 0% to 100% open in less than 40 seconds. In other embodiments,the variable position starter valve 116B has a response rate of 0% to100% open in less than 30 seconds. In further embodiments, the variableposition starter valve 116B has a response rate of 0% to 100% open inless than 20 seconds.

The controller 102 can monitor a valve angle of the variable positionstarter valve 116B using valve angle feedback signals 152 provided toboth channels of controller 102. As one example, in an active/standbyconfiguration, both channels of the controller 102 can use the valveangle feedback signals 152 to track a current valve angle, while onlyone channel designated as an active channel outputs valve control signal150. Upon a failure of the active channel, the standby channel ofcontroller 102 can take over as the active channel to output valvecontrol signal 150. In an alternate embodiment, both channels ofcontroller 102 output all or a portion of a valve angle commandsimultaneously on the valve control signals 150. The controller 102 canestablish an outer control loop with respect to rotor speed and an innercontrol loop with respect to the valve angle of the variable positionstarter valve 116B.

As in the example of FIG. 2, the starting system 100A of FIG. 3 alsoincludes vibration monitoring system 126. The vibration monitoringsystem 126 includes at least one vibration pickup 128, e.g., anaccelerometer, operable to monitor vibration of the gas turbine engine10 of FIG. 1. Vibration signal processing 130 can be performed locallywith respect to the vibration pickup 128, within the controller 102, orthrough a separate vibration processing system, which may be part of anengine health monitoring system to acquire vibration data 132.Alternatively, the vibration monitoring system 126 can be omitted insome embodiments.

FIG. 4 is a block diagram of a system 200 for bowed rotor startmitigation that may control the discrete starter valve 116A of FIG. 2 orthe variable position starter valve 116B of FIG. 3 via control signals210 in accordance with an embodiment. The system 200 may also bereferred to as a bowed rotor start mitigation system. In the example ofFIG. 4, the system 200 includes an onboard model 202 operable to producea compressor exit temperature T₃ and a compressor inlet flow W₂₅ for useby a core temperature model 204. The onboard model 202 is configured tosynthesize or predict major temperatures and pressures throughout thegas turbine engine 10 of FIG. 1 beyond those sensed by sensorspositioned about the gas turbine engine 10. The onboard model 202 andcore temperature model 204 are examples of a first thermal model and asecond thermal model that may be separately implemented or combined aspart of controller 102 and resident upon an electronic engine controlsystem of the gas turbine engine 10 of FIG. 1.

Engine parameter synthesis is performed by the onboard model 202, andthe engine parameter synthesis may be performed using the technologiesdescribed in U.S. Patent Publication No. 2011/0077783, the entirecontents of which are incorporated herein by reference thereto. Of themany parameters synthesized by onboard model 202 at least two areoutputted to the core temperature model 204, T₃, which is the compressorexit gas temperature of the engine 10 and W₂₅, which is the air flowthrough the compressor. Each of these values are synthesized by onboardmodel 202 and inputted into the core temperature model 204 thatsynthesizes or provides a heat state (T_(core)) of the gas turbineengine 10. T_(core) can be determined by a first order lag or functionof T₃ and a numerical value X (e.g., f(T₃, X)), wherein X is a valuedetermined from a lookup table stored in memory of controller 102.Accordingly, X is dependent upon the synthesized value of W₂₅. In otherwords, W₂₅ when compared to a lookup table of the core temperature model204 will determine a value X to be used in determining the heat state orT_(core) of the engine 10. In one embodiment, the higher the value ofW₂₅ or the higher the flow rate through the compressor the lower thevalue of X.

The heat state of the engine 10 during use or T_(core) is determined orsynthesized by the core temperature model 204 as the engine 10 is beingrun. In addition, T₃ and W₂₅ are determined or synthesized by theonboard model 202 and/or the controller 102 as the engine 10 is beingoperated.

At engine shutdown, the current or most recently determined heat stateof the engine or T_(core shutdown) of the engine 10 is recorded into DSU104, and the time of the engine shutdown t_(shutdown) is recorded intothe DSU 104. Time values and other parameters may be received oncommunication link 106. As long as electrical power is present for thecontroller 102 and DSU 104, additional values of temperature data may bemonitored for comparison with modeled temperature data to validate oneor more temperature models (e.g., onboard model 202 and/or coretemperature model 204) of the gas turbine engine 10.

During an engine start sequence or restart sequence, a bowed rotor startrisk model 206 (also referred to as risk model 206) of the controller102 is provided with the data stored in the DSU 104, namelyT_(core shutdown) and the time of the engine shutdown t_(shutdown). Inaddition, the bowed rotor start risk model 206 is also provided with thetime of engine start t_(start) and the ambient temperature of the airprovided to the inlet of the engine 10 T_(inlet) or T₂. T₂ is a sensedvalue as opposed to the synthesized value of T₃.

The bowed rotor start risk model 206 maps core temperature model datawith time data and ambient temperature data to establish a motoring timet_(motoring) as an estimated period of motoring to mitigate a bowedrotor of the gas turbine engine 10. The motoring time t_(motoring) isindicative of a bowed rotor risk parameter computed by the bowed rotorstart risk model 206. For example, a higher risk of a bowed rotor mayresult in a longer duration of dry motoring to reduce a temperaturegradient prior to starting the gas turbine engine 10 of FIG. 1. As willbe discussed herein and in one embodiment, an engine start sequence mayautomatically include a modified start sequence; however, the durationof the modified start sequence prior to a normal start sequence willvary based upon the time period t_(motoring) that is calculated by thebowed rotor start risk model 206. The motoring time t_(motoring) forpredetermined target speed N_(target) of the engine 10 is calculated asa function of T_(core shutdown), t_(shutdown), t_(start) and T₂, (e.g.,f(T_(core shutdown), t_(shutdown), t_(start) and T₂), while a targetspeed N_(target) is a predetermined speed that can be fixed or varywithin a predetermined speed range of N_(targetMin) to N_(targetMax). Inother words, the target speed N_(target) may be the same regardless ofthe calculated time period t_(motoring) or may vary within thepredetermined speed range of N_(targetMin) to N_(targetMax). The targetspeed N_(target) may also be referred to as a dry motoring mode speed.

Based upon these values (T_(core shutdown), t_(shutdown), t_(start) andT₂) the motoring time t_(motoring) at a predetermined target speedN_(target) for the modified start sequence of the engine 10 isdetermined by the bowed rotor start risk model 206. Based upon thecalculated time period t_(motoring) which is calculated as a time to runthe engine 10 at a predetermined target speed N_(target) in order toclear a “bowed condition”. In accordance with an embodiment of thedisclosure, the controller 102 can run through a modified start sequenceupon a start command given to the engine 10 by an operator of the engine10 such as a pilot of an airplane the engine is used with. It isunderstood that the motoring time t_(motoring) of the modified startsequence may be in a range of 0 seconds to minutes, which depends on thevalues of T_(core shutdown), t_(shutdown), t_(start) and T₂.

In an alternate embodiment, the modified start sequence may only be runwhen the bowed rotor start risk model 206 has determined that themotoring time t_(motoring) is greater than zero seconds upon receipt ofa start command given to the engine 10. In this embodiment and if thebowed rotor start risk model 206 has determined that t_(motoring) is notgreater than zero seconds, a normal start sequence will be initiatedupon receipt of a start command to the engine 10.

Accordingly and during an engine command start, the bowed rotor startrisk model 206 of the system 200 may be referenced wherein the bowedrotor start risk model 206 correlates the elapsed time since the lastengine shutdown time and the shutdown heat state of the engine 10 aswell as the current start time t_(start) and the inlet air temperatureT₂ in order to determine the duration of the modified start sequencewherein motoring of the engine 10 at a reduced speed N_(target) withoutfuel and ignition is required. As used herein, motoring of the engine 10in a modified start sequence refers to the turning of a starting spoolby the starter 120 at a reduced speed N_(target) without introduction offuel and an ignition source in order to cool the engine 10 to a pointwherein a normal start sequence can be implemented without starting theengine 10 in a bowed rotor state. In other words, cool or ambient air isdrawn into the engine 10 while motoring the engine 10 at a reduced speedin order to clear the “bowed rotor” condition, which is referred to as adry motoring mode.

The bowed rotor start risk model 206 can output the motoring timet_(motoring) to a motoring controller 208. The motoring controller 208uses a dynamic control calculation in order to determine a requiredvalve position of the starter valve 116A, 116B used to supply an airsupply or starter air supply 114 to the engine 10 in order to limit themotoring speed of the engine 10 to the target speed N_(target) due tothe position of the starter valve 116A, 116B. The required valveposition of the starter valve 116A, 116B can be determined based upon anair supply pressure as well as other factors including but not limitedto ambient air temperature, parasitic drag on the engine 10 from avariety of engine driven components such as electric generators andhydraulic pumps, and other variables such that the motoring controller208 closes the loop for an engine motoring speed target N_(target) forthe required amount of time based on the output of the bowed rotor startrisk model 206. In one embodiment, the dynamic control of the valveposition (e.g., open state of the valve (e.g., fully open, ½ open, ¼open, etc.) in order to limit the motoring speed of the engine 10) iscontrolled via duty cycle control (on/off timing using pulse widthmodulation) of electromechanical device 110 for discrete starter valve116A.

When the variable position starter valve 116B of FIG. 3 is used, a valveangle 207 can be provided to motoring control 208 based on the valveangle feedback 152 of FIG. 3. A rotor speed N2 (i.e., speed of highspeed spool 32) can be provided to the motoring controller 208 and amitigation monitor 214, where motoring controller 208 and a mitigationmonitor 214 may be part of controller 102. Vibration data 132 can alsobe provided to mitigation monitor 214.

The risk model 206 can determine a bowed rotor risk parameter that isbased on the heat stored (T_(core)) using a mapping function or lookuptable. When not implemented as a fixed rotor speed, the bowed rotor riskparameter can have an associated dry motoring profile defining a targetrotor speed profile over an anticipated amount of time for the motoringcontroller 208 to send control signals 210, such as valve controlsignals 150 for controlling variable position starter valve 116B of FIG.3.

The bowed rotor risk parameter may be quantified according to a profilecurve 402 selected from a family of curves 404 that align with observedaircraft/engine conditions that impact turbine bore temperature and theresulting bowed rotor risk as depicted in the example graph 400 of FIG.5.

In some embodiments, an anticipated amount of dry motoring time can beused to determine a target rotor speed profile in a dry motoring profilefor the currently observed conditions. As one example, one or morebaseline characteristic curves for the target rotor speed profile can bedefined in tables or according to functions that may be rescaled toalign with the observed conditions. An example of a target rotor speedprofile 1002 is depicted in graph 1000 of FIG. 12 that includes a steepinitial transition portion 1004, followed by a gradually increasingportion 1006, and a late acceleration portion 1008 that increases rotorspeed above a critical rotor speed, through a fuel-on speed and anengine idle speed. The target rotor speed profile 1002 can be rescaledwith respect to time and/or select portions (e.g., portions 1004, 1006,1008) of the target rotor speed profile 1002 can be individually orcollectively rescaled (e.g., slope changes) with respect to time toextend or reduce the total motoring time. The target rotor speed profile1002 may include all positive slope values such that the actual rotorspeed 1010 is driven to essentially increase continuously while bowedrotor start mitigation is active. While the example of FIG. 12 depictsone example of the target rotor speed profile 1002 that can be definedin a dry motoring profile, it will be understood that many variationsare possible in embodiments.

An example of the effects of bowed rotor mitigation are illustrated ingraph 420 of FIG. 7 that depicts a normal or cooled engine start (line432) versus a bowed rotor or mitigated engine start (line 434) inaccordance with one non-limiting embodiment of the disclosure. At point436, a pilot or operator of the engine 10 sets or initiates a startcommand of the engine. At point 438 and after the start command isinitiated, the controller 102, based upon the risk model 206, requiresthe engine to motor at a pre-determined speed (N_(pre-determined) orN_(target)), which is less than a normal idle start speed N2 for a time(t_(determined)). The pre-determined speed (N_(pre-determined) orN_(target)) can be defined within a predetermined speed rangeN_(targetMin) to N_(targetMax) that is used regardless of the calculatedtime period t_(motoring) for homogenizing engine temperatures. The timeperiod t_(determined) is based upon the output of the risk model 206.The determined speed (N_(pre-determined) or N_(target)) is achieved bycontrolling the operational position of starter valve 116A, 116B.Thereafter and at point 440 when the required motoring time (determinedfrom the risk model 206) has been achieved, such that the “bowedcondition” has been cleared a normal start sequence with a normal speedN2 is initiated. Subsequently and at point 442, the idle speed N2 hasbeen achieved. This modified sequence is illustrated in one non-limitingmanner by the dashed line 434 of the graph 420 of FIG. 7. It is, ofcourse, understood that (t_(determined)) may vary depending upon theoutputs of the risk model 206, while N_(pre-determined) or N_(target) isa known value. Of course, in alternative embodiments, the risk model 206may be configured to provide the speed of the engine 10 during amodified start sequence. Still further and as mentioned above, thestarter valve may be dynamically varied based upon the outputs of therisk model 206 as well as the pressure of the air supply 114 in order tolimit the motoring speed of the engine 10 to that of N_(pre-determined)or N_(target) during the clearing of a bowed rotor condition. Line 432illustrates a normal start sequence wherein the time t_(determined) iszero for a modified start as determined by the risk model 206.

The example of FIG. 11 illustrates how a valve angle command 902 can beadjusted between 0 to 100% of a commanded starter valve opening togenerate the actual rotor speed 1010 of FIG. 12. As the actual rotorspeed 1010 tracks to the steep initial transition portion 1004 of thetarget rotor speed profile 1002, the valve angle command 902 transitionsthrough points “a” and “b” to fully open the variable position startervalve 116B. As the slope of the target rotor speed profile 1002 isreduced in the gradually increasing portion 1006, the valve anglecommand 902 is reduced between points “b” and “c” to prevent the actualrotor speed 1010 from overshooting the target rotor speed profile 1002.In some embodiments, decisions to increase or decrease the commandedstarter valve opening is based on monitoring a rate of change of theactual rotor speed 1010 and projecting whether the actual rotor speed1010 will align with the target rotor speed profile 1002 at a futuretime. If it is determined that the actual rotor speed 1010 will notalign with the target rotor speed profile 1002 at a future time, thenthe valve angle of the variable position starter valve 116B is adjusted(e.g., increase or decrease the valve angle command 902) at acorresponding time. In the example of FIGS. 11 and 12, the valve anglecommand 902 oscillates with a gradually reduced amplitude between points“c”, “d”, and “e” as the actual rotor speed 1010 tracks to the targetrotor speed profile 1002 through the gradually increasing portion 1006.As dry motoring continues, the overall homogenization of the engine 10increases, which allows the actual rotor speed 1010 to safely approachthe critical rotor speed without risking damage. The valve angle commandtransitions from point “e” to point “f” and beyond to further increasethe actual rotor speed 1010 in the late acceleration portion 1008 abovethe critical rotor speed, through a fuel-on speed and an engine idlespeed. By continuously increasing the actual rotor speed 1010 during drymotoring, the bowed rotor condition can be reduced faster than holding aconstant slower speed.

In summary with reference to FIG. 4, as one example of an aircraft thatincludes systems as described herein, onboard model 202 and coretemperature model 204 may run on controller 102 of the aircraft to trackheat stored (T_(core)) in the turbine at the time of engine shutdown.Modeling of potential heat stored in the system may be performed as aturbine disk metal temperature model in the core temperature model 204.When the aircraft lands, engines typically operate at idle for a cooldown period of time, e.g., while taxiing to a final destination. When anengine shutdown is detected, model state data can be logged by the DSU104 prior to depowering. When the controller 102 powers on at a latertime and model state data can be retrieved from the DSU 104, and thebowed rotor start risk model 206 can be updated to account for theelapsed time. When an engine start is requested, a bowed rotor risk canbe assessed with respect to the bowed rotor start risk model 206.Extended dry motoring can be performed during an engine start processuntil the bow risk has sufficiently diminished. Peak vibrations can bechecked by the mitigation monitor 214 during the start processes toconfirm that bowed rotor mitigation successfully removed the bowed rotorcondition.

In reference to FIGS. 4 and 12, the mitigation monitor 214 of FIG. 4 canoperate in response to receiving a complete indicator 212 to run averification of the bowed rotor mitigation. The mitigation monitor 214can provide mitigation results 216 to the motoring controller 208 andmay provide result metrics 218 to other systems, such a maintenancerequest or indicator. Peak vibrations can be checked by the mitigationmonitor 214 during the start processes to confirm that bowed rotormitigation successfully removed the bowed rotor condition. Themitigation monitor 214 may also run while dry motoring is active todetermine whether adjustments to the dry motoring profile are needed.For example, if a greater amount of vibration is detected than wasexpected, the mitigation monitor 214 can request that the motoringcontroller 208 reduce a slope of the target rotor speed profile 1002 ofFIG. 12 to extend the dry motoring time before driving the actual rotorspeed 1010 of FIG. 12 up to the critical rotor speed. Similarly, if themagnitude of vibration observed by the mitigation monitor 214 is lessthan expected, the mitigation monitor 214 can request that the motoringcontroller 208 increase a slope of the target rotor speed profile 1002of FIG. 12 to reduce the dry motoring time before driving the actualrotor speed 1010 of FIG. 12 up to the critical rotor speed.

FIG. 5 is a flow chart illustrating a method 300 of bowed rotor startmitigation of the gas turbine engine 10 in accordance with anembodiment. The method 300 of FIG. 5 is described in reference to FIGS.1-12 and may be performed with an alternate order and include additionalsteps. Before initiating bowed rotor start mitigation, a bowed rotordetermination step can be performed to estimate a need for bowed rotorstart mitigation. Examples include the use of models and/orstored/observed engine/aircraft state data, such as data received fromDSU 104, communication link 106, and/or reading data from one or moretemperature sensors of the gas turbine engine 10.

At block 302, the controller 102 determines a heat state (T_(core)) ofthe gas turbine engine 10 via an engine thermal model (e.g., onboardmodel 202 and core temperature model 204 of FIG. 4). At block 304, thecontroller 102 stores the heat state of the gas turbine engine atshutdown into DSU 104. At block 306, the controller 102 records a timeof the engine shutdown in DSU 104.

At block 308, the controller 102 uses risk model 206 to determine amotoring time period t_(motoring) for a start sequence of the gasturbine engine 10, where the risk model 206 uses the recorded time ofthe engine shutdown and the stored heat state of the gas turbine engine10 at shut down to determine the motoring time period t_(motoring). Thegas turbine engine 10 is motored at a predetermined speed range ofN_(targetMin) to N_(targetMax) during the motoring time period, which isless than a normal idle start speed N2. The controller 102 candynamically vary a position of starter valve 116A, 116B during themotoring time period in order to motor the gas turbine engine 10 at thepredetermined speed range of N_(targetMin) to N_(targetMax). Thepredetermined speed range of N_(targetMin) to N_(targetMax) may betightly controlled to a substantially constant rotor speed or cover awider operating range according to a dry motoring profile.

As one example with respect to FIGS. 3 and 12, the variable positionstarter valve 116B can be initially set to a valve angle of greater than50% open when bowed rotor start mitigation is active. The controller 102can monitor a rate of change of the actual rotor speed 1010, projectwhether the actual rotor speed 1010 will align with the target rotorspeed profile 1002 at a future time based on the rate of change of theactual rotor speed 1010, and adjust a valve angle of the variableposition starter valve 116B based on determining that the actual rotorspeed 1010 will not align with the target rotor speed profile 1002 at afuture time.

Further dynamic updates at runtime can include adjusting a slope of thetarget rotor speed profile 1002 in the dry motoring profile while thebowed rotor start mitigation is active based on determining that avibration level of the gas turbine engine 10 is outside of an expectedrange. Adjusting the slope of the target rotor speed profile 1002 caninclude maintaining a positive slope. Vibration levels may also oralternatively be used to check/confirm successful completion of bowedrotor start mitigation prior to starting the gas turbine engine 10. Forinstance, based on determining that the bowed rotor start mitigation iscomplete, a vibration level of the gas turbine engine 10 can bemonitored while sweeping through a range of rotor speeds including thecritical rotor speed.

In further reference to FIG. 4, the mitigation monitor 214 of FIG. 4 mayreceive a complete indicator 212 from the motoring controller 208 whenthe motoring controller 208 has completed dry motoring, for instance, ifthe motoring time has elapsed. If the mitigation monitor 214 determinesthat the bowed rotor condition still exists based on vibration data 132collected, the motoring controller 208 may restart dry motoring, or amaintenance request or indicator can be triggered along with providingresult metrics 218 for further analysis. Metrics of attempted bowedrotor mitigation can be recorded in the DSU 104 based on determiningthat the attempted bowed rotor mitigation was unsuccessful orincomplete.

Referring now to FIG. 8, a graph 500 illustrating examples of variousvibration level profiles 502 of an engine, such as gas turbine engine 10of FIG. 1 is depicted. The vibration level profiles 502 represent avariety of possible vibration levels observed before and/or afterperforming bowed rotor mitigation. Critical rotor speed 510 is the speedat which a vibration peak is expected due to amplification effects of abowed rotor condition along with other contributions to vibration levelgenerally. A peak vibration 504 at critical rotor speed 510 may be usedto trigger different events. For example, if the peak vibration 504 atcritical rotor speed 510 is below a maintenance action threshold 506,then no further actions may be needed. If the peak vibration 504 atcritical rotor speed 510 is above a damage risk threshold 508, then anurgent maintenance action may be requested such as an engine check. Ifthe peak vibration 504 at critical rotor speed 510 is between themaintenance action threshold 506 and the damage risk threshold 508, thenfurther bowed rotor mitigation actions may be requested, such asextending/restarting dry motoring. In one embodiment, a maintenancerequest is triggered based on the actual vibration level exceedingmaintenance action threshold 506 after completing an attempt of bowedrotor mitigation.

The lowest rotor vibration vs. speed in FIG. 8 (vibration profile 502D)is for a fully homogenized rotor, where mitigation is not necessary(engine parked all night long, for example). The next higher curve showsa mildly bowed rotor and so on. The maintenance action threshold 506 isa threshold for setting a maintenance flag such as requiring atroubleshooting routine of one or more system elements. The damage riskthreshold 508 may be a threshold to trigger a more urgent maintenancerequirement up to and including an engine check. As dry motoring isperformed in embodiments, the gas turbine engine 10 may shift betweenvibration profiles. For instance, when a bow rotor condition is present,the gas turbine engine 10 may experience vibration levels according tovibration profile 502A, if mitigation is not performed. As dry motoringis run, the gas turbine engine 10 may have a vibration profile that isgradually reduced from vibration profile 502A to vibration profile 502Band then vibration profile 502C, for example. By checking the currentvibration level at a corresponding rotor speed with respect to time, thecontroller 102 can determine whether adjustments are needed to extend orreduce the slope of the target rotor speed profile 1002 of FIG. 12depending on an expected rate of bowed rotor reduction. In embodiments,a slope of the target rotor speed profile 1002 in the dry motoringprofile 206 can be adjusted and maintains a positive slope while bowedrotor start mitigation is active based on determining that a vibrationlevel of the gas turbine engine 10 is less than a targeted maximum range512, which may define a safe level of vibration to ensure that no riskof a maintenance action or damage will likely occur if the actual rotorspeed 1010 is increased faster than previously planned.

Accordingly and as mentioned above, it is desirable to detect, preventand/or clear a “bowed rotor” condition in a gas turbine engine that mayoccur after the engine has been shut down. As described herein and inone non-limiting embodiment, the controller 102 may be programmed toautomatically take the necessary measures in order to provide for amodified start sequence without pilot intervention other than theinitial start request. In an exemplary embodiment, the controller 102and/or DSU 104 comprises a microprocessor, microcontroller or otherequivalent processing device capable of executing commands of computerreadable data or program for executing a control algorithm and/oralgorithms that control the start sequence of the gas turbine engine. Inorder to perform the prescribed functions and desired processing, aswell as the computations therefore (e.g., the execution of Fourieranalysis algorithm(s), the control processes prescribed herein, and thelike), the controller 102 and/or DSU 104 may include, but not be limitedto, a processor(s), computer(s), memory, storage, register(s), timing,interrupt(s), communication interfaces, and input/output signalinterfaces, as well as combinations comprising at least one of theforegoing. For example, the controller 102 and/or DSU 104 may includeinput signal filtering to enable accurate sampling and conversion oracquisitions of such signals from communications interfaces. Asdescribed above, exemplary embodiments of the disclosure can beimplemented through computer-implemented processes and apparatuses forpracticing those processes.

While the present disclosure has been described in detail in connectionwith only a limited number of embodiments, it should be readilyunderstood that the present disclosure is not limited to such disclosedembodiments. Rather, the present disclosure can be modified toincorporate any number of variations, alterations, substitutions orequivalent arrangements not heretofore described, but which arecommensurate with the spirit and scope of the present disclosure.Additionally, while various embodiments of the present disclosure havebeen described, it is to be understood that aspects of the presentdisclosure may include only some of the described embodiments.Accordingly, the present disclosure is not to be seen as limited by theforegoing description, but is only limited by the scope of the appendedclaims.

The invention claimed is:
 1. A system for starting a gas turbine engineof an aircraft, the system comprising: a pneumatic starter motor; adiscrete starter valve switchable between an on-state and an off-state;a controller operable to perform a starting sequence for the gas turbineengine, the starting sequence comprising alternating on and off commandsto an electromechanical device coupled to the discrete starter valve toachieve a partially open position of the discrete starter valve tocontrol a flow from a starter air supply to the pneumatic starter motorto drive rotation of a starting spool of the gas turbine engine below anengine idle speed; an electronic engine control system comprising: amemory for recording a current heat state of the gas turbine engine atshutdown and for recording a shutdown time of the gas turbine engine;and a risk model for determining a time period (t_(motoring)) formotoring the gas turbine engine at a predetermined speed N_(target)wherein the predetermined speed is less than a speed used to start thegas turbine engine and wherein t_(motoring) is a function of the heatstate recorded at engine shutdown and an elapsed time of an engine startrequest relative to the previous shutdown time.
 2. The system as inclaim 1, wherein the electromechanical device has a cycle time definedbetween an off-command to an on-command to the off-command that is atmost half of a movement time for the discrete starter valve totransition from fully closed to fully open.
 3. The system as in claim 1,wherein the electromechanical device is a solenoid that positions thediscrete starter valve based on intermittently supplied electric power.4. The system as in claim 1, wherein the electromechanical device is anelectric valve controlling muscle air to adjust the position of thediscrete starter valve.
 5. The system as in claim 1, wherein thecontroller modulates the on and off commands to the electromechanicaldevice to further open the discrete starter valve and increase arotational speed of the starting spool.
 6. The system as in claim 1,wherein the time period (t_(motoring)) is calculated automaticallyduring a start of the gas turbine engine.
 7. The system as in claim 1,wherein the controller modulates a duty cycle of the discrete startervalve via pulse width modulation.
 8. The system as in claim 1, whereinthe predetermined speed N_(target) is within a predetermined speed rangeN_(targetMin) to N_(targetMax) that is used regardless of the calculatedtime period t_(motoring).